that's correct. what order have you configured in the 'dataset merge precedence set' ..? precedence group for BMC.ASSET should have order 1 and precedence group for BMC.ADDM should have order 2
have a look at this discussion for even more clarity
Could you please set the job in debug and send the job log. And also highlight the name of the custom attribute?
Would like to see the precedence values for ADDM dataset, the custom attribute in the ADDM dataset, and the custom attribute in Asset dataset
All these values show up in debug job logs.